Most military industrial enterprises in China use epoxy insulation varnish as a three-anticoating material, using traditional immersion, brushing and spraying processes. Because the output of military products is relatively small, many companies use manual operations. The main disadvantage of manual operation is that it is not easy to grasp the thickness of the coating layer, the uniformity is poor, and the consistency is poor; the problem of masking is usually masked with adhesive tape, but it is not easy to solve for the large abnormal shaped parts; in addition, the manual operation is relatively large. The pollution is bad for the operator and the environment. With the development of science and technology, many military enterprises have purchased selective coating equipment and adopted automatic selective coating process.
Whether it is manual immersion, brushing, spraying, or selective coating, the process is the same. The process flow is as follows:
Clean PCB assembly board after PCB welding → cleanliness testing → drying → coating of three anti-paints → curing → testing and maintenance.
The three-anti process should also pay special attention to process control and process continuity: the entire PCB welding, cleaning, drying and three-anti process should be arranged in one line. Doing it together is good for improving the quality of pcb and the pass rate of qualified products. Because the air dust, micro-creation, moisture (especially high humidity in summer air) have an impact on quality.
(1) PCB assembly board cleaning
The purpose of cleaning is to remove the residues of solder and flux between the surface of the circuit board and device, the bottom, the vias and pins, and the pollutants brought in during the PCB manufacturing process to avoid potential corrosion risks, while improving the PCBA bond strength.
The cleaning methods include solvent cleaning, water cleaning and semi-water cleaning. Which method is used depends on the reliability requirements of the electronic product, the properties of the flux used during soldering, and the specific conditions of the residue. In general, for military products and products with high reliability requirements, water-soluble flux is used for welding, and water cleaning is the best after welding.
The weldability of water-soluble flux is very good, but it has a corrosive effect on the PCB solder joints. It must be cleaned immediately after soldering (up to 2 hours), and the PCB board is repaired immediately after cleaning (using water-soluble flux and water-soluble solder wire) The repaired assembly board must be cleaned within 2h.
The detection of PCBA cleanliness generally requires the measurement of Na ion contamination on the cleaned PCB using measuring instruments such as an omega (Ω) meter; in addition, comb-shaped test pieces are usually used to test the surface insulation resistance.
Questions about circuit board no-clean:
No-cleaning is not without residues, but with less residues, does not absorb water, and the insulation resistance meets the requirements. However, some domestic companies have not strictly controlled the quality of welding materials, and the no-clean process conditions and process control have failed to meet the requirements, resulting in the use of no-clean, which requires cleaning, and the result is not clean. It’s like spraying paint on a wall that has not been cleaned or smoothed. The effect after painting can be imagined. Therefore, under the current conditions, it is recommended that the three-anti electronic products adopt a cleaning process.
Drying after water washing is very important for the three-anti process. Incomplete drying will affect the quality of the three defenses. (3) Coating of three-anti paint
The coating mainly controls the thickness, uniformity and compactness of the coating.
Automatic coating equipment is used to apply the three-anti coating to the surface of the circuit board efficiently and uniformly to form a protective film. Automated equipment can improve coating uniformity and consistency, reduce paint waste and environmental pollution
Need to choose thermal curing or UV curing equipment according to the type of coating.
It is required to dry and solidify the coating in a short time to form a coating film. The curing conditions of most thermosetting coating materials are 5 to 10 minutes at 120 ° C; UV (ultraviolet) curing requires that the time of exposure to ultraviolet rays is controlled to 10 to 20s, and the curing conditions are 5 to 10 minutes at 120 ° C. It should be noted that some coatings will release flammable gases during curing. Anti-explosionissues must be considered.
Three-dimensional oven can be selected for small-batch PCB production; most of the large-scale production uses a flow-type curing furnace with the same structure as an air reflow furnace.
(4) Generally,Inspection and maintenance adjust the process parameters of the coating process by testing the thickness, uniformity and compactness of the coating to achieve relevant standards. At the same time, the coatings that can not meet the requirements are removed or repaired according to the test results. In addition, repairs also include component adjustments, tests, and replacements for those PCBAs that do not meet the performance specifications.
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